Peripheral neuropathy is the principal dose-limiting factor for each of the major frontline chemotherapeutic drugs used against all the most common types of cancer and hence affects hundreds of thousands of patients each year. Neuropathy causes such distress that many patients will drop out of potentially curative therapy, directly impacting their survival. Chemotherapy-induced peripheral neuropathy (CIPN) is refractory to treatment and often persists in cancer survivors limiting quality of life, rehabilitation and the return to productivity. The continuing long-term goal of this project is to determine the mechanisms of CIPN and identify potential therapeutic interventions for its relief or prevention. The main hypothesis in this project is that chemotherapeutics activate toll-like receptor 4 (TLR4) on subsets of DRG neurons that 1) trigger altered cellular signaling in DRG neurons and infiltration of the DRG by immune cells that initiate and sustain CIPN; 2) drive the loss of distal epidermal nerve fibers (ENFs) to a critical point associated with onset of CIPN symptoms; and 3) cause activation of spinal glia and neurons that result in maladaptive changes in the physiology of these cells that further contribute to CIPN symptoms. This hypothesis will be tested in humans and animals that receive chemotherapy treatments. Specific Aim 1 will determine whether activation of innate immunity in the DRG is a key initiating step in CIPN. Specific Aim 2 will determine whether CIPN in humans is triggered when peripheral innervation density decreases to a critical point during chemotherapy. Specific Aim 3 will determine the role of TLR4 and damage associated molecular pattern (DAMP) proteins in driving maladaptive plasticity in the spinal dorsal horn contributing to CIPN. In summary this project will define mechanisms of chemotherapy-induced peripheral neuropathy, identify patient risk factors and potential new near-term protective and treatment candidates. This project will therefore impact on the quality of life, survival and the return to productivity of thousands of cancer patients.